The present disclosure relates to fixing devices configured to fix a toner image on a sheet and image forming apparatuses to which the fixing devices are applied.
A general fixing device for an image forming apparatus includes a fixing nip formed by a fixing roller and a pressure roller pressed against each other. A sheet to which an unfixed toner image is transferred is nipped by the fixing nip and conveyed downstream by the rotating fixing roller and pressure roller. While the sheet passes through the fixing nip, pressure and heat are applied to the sheet, so that the toner image is fixed on the sheet. Generally, the fixing roller is equipped with a heat source, such as an electric heater or an IH heater, capable of generating heat necessary for toner fixation. This heat is given through the fixing nip to the pressure roller. The pressure roller is driven into rotation and the fixing roller rotates to follow the rotation of the pressure roller.
In order to form an image of good quality, it is necessary to maintain the conveyance speed of the sheet passing through the fixing nip constant. However, the sheet conveyance characteristics at the fixing nip varies depending upon factors associated with changes in roller temperature, such as changes in roller diameter. As a solution to this problem, there is known a fixing device configured to detect the surface temperature of the roller using a temperature sensor and control the rotational speed of the roller by feedback based on the detected temperature.